1. Field of the Invention
The present invention relates in general to micro-roll forming devices that form a minute convexoconcave bearing surface on a cylindrical metal article such as a camshaft, a piston or the like for providing the bearing portion of the article with a sufficiently low friction.
2. Description of the Related Art
In order to clarify the task of the present invention, one conventional device for providing a cylindrical metal article with a minute convexoconcave bearing surface will be briefly described with reference to FIGS. 10A and 10B of the accompanying drawings, which is shown in Japanese Laid-open Patent Application (Tokkai) 2000-227119.
As is seen from FIGS. 10A and 10B, the device is a shot peening device. In operation, a masking sheet 101 having a plurality of fine openings 101a is pasted on a cylindrical outer wall of a metal work piece “W”, and then, with the work piece “W” being rotated, fine hard balls “P” of steel or ceramics are blown against masking sheet 101 together with a blast of air from an injection nozzle 102. With this, portions of the cylindrical outer wall of work piece “W” that are exposed to fine openings 101a are formed with a plurality of fine recesses, so that work piece “W” has a herringbone type grooved surface on the cylindrical outer wall thereof.